Not Applicable.
Not applicable.
The present invention generally relates to coherent detection. More specifically, the present invention relates to tracking the absolute phase of a received waveform for coherent detection in a phase or frequency modulated communication system where the transmitted waveform consists of both phase reference symbols and data symbols.
Phase modulation (xe2x80x9cPMxe2x80x9d) is widely used in communication systems. In phase modulation schemes, data is represented by either the absolute phase of the waveform or by changes in the phase of the waveform. One reason for the popularity of PM is the robustness of PM with respect to additive white Gaussian noise. Common examples include Binary Phase Shift Keying (xe2x80x9cBPSKxe2x80x9d), Quadrature Phase Shift Keying (xe2x80x9cQPSKxe2x80x9d), and Gaussian Minimum Shift Keying (xe2x80x9cGMSKxe2x80x9d). QPSK, for example, represents two bits of information (which may assume a total of four different values) in every symbol. A symbol consists of a phase shift by one of four possible phase shift values. The phase shift values are typically chosen to be plus/minus 45 degrees and plus/minus 135 degrees.
As the demand for communication bandwidth rises, the concern over efficient use of available bandwidth similarly rises. GMSK has been chosen by many because of its relatively efficient use of bandwidth. In GMSK, symbols are represented by gradual changes in phase which result in a power spectral density that rapidly falls off. This allows GMSK channels to be packed relatively efficiently into a given frequency band.
The scenario of interest involves modulation schemes that map information to the absolute phase of the waveform. As such, phase shift values are measured with respect to some reference. In order for a receiver to extract data from the received waveform, the phase shift values relative to the reference must be known. A receiver, therefore, needs to have knowledge of the phase reference. The transmitted waveform may contain symbols whose express purpose is to provide the receiver with explicit phase reference. For example, the GMSK waveform, as defined in the Advanced EHF Waveform Functional Description includes a specification for transmitting both phase reference symbols and data symbols. A receiver may then extract the absolute phase directly from the phase reference symbols.
Typically, phase lock loops or similar circuits with local oscillators are utilized in the process of signal channelization and detection. In some cases, the local oscillators may be used to provide phase references to data detection circuitry. Data detection circuitry may even be able to assert control over the local oscillators once more is known about the true phase reference. However, access to local oscillators may not always be practical. In some communications systems, the local oscillators may be set to specific frequencies or may be too slow to react to incoming phase reference information. In addition, the local oscillators may be used exclusively to extract the baseband waveform from an intermediate frequency, while the task of extracting both phase reference and data decisions from the baseband waveform falls on independent circuitry. To this end, methods have been developed to determine the absolute phase of the received waveform based on phase reference symbols. However, the methods that have been developed thus far focus on determining the absolute phase using only the phase reference symbols. Such methods fail to take advantage of additional phase reference information that may be obtained from the data symbols, and thus do not track the absolute phase of the received waveform as well as they potentially could.
A need exists for a coherent detection system that is capable of achieving a better phase reference than systems which utilize only phase reference symbols.
It is an object of the present invention to provide an improved scheme for tracking the absolute phase of a received waveform in a communication system.
It is another object of the present invention to provide an improved scheme for tracking the absolute phase of a received waveform in a coherent detection system which utilizes both phase reference symbols and data symbols contained in the waveform.
It is still another object of the present invention to provide an improved scheme for tracking the absolute phase of a received waveform in a coherent detection system in which a nominal waveform is generated from the detected data and is fed back and compared to the received waveform to determine the necessary amount of phase correction.
A preferred embodiment of the present invention provides a method and apparatus for coherent detection including tracking the phase of a received waveform containing both phase reference symbols and data symbols. The received waveform is phase-rotated by a phase offset estimate. Data decisions are extracted from the phase-corrected version of the received waveform. A nominal waveform is generated based on the data decisions resulting from the phase-corrected version of the received waveform. A phase measurement is generated from the difference in phase between the received waveform and the nominal waveform. The received waveform is phase-rotated by the phase offset estimate which is calculated from the phase measurements.
The apparatus provides a phase rotator for phase-rotating the received waveform by the phase offset estimate. A data detector is provided which extracts data decisions from the phase-corrected version of the received waveform. A waveform modulator is provided to generate a nominal waveform based on the data decisions generated from the data detector. A buffer is provided to time-align the received waveform with the nominal waveform. A phase error signal calculator is provided to generate a measurement of the difference between the phase of the nominal waveform and the phase of the received waveform. A phase correction angle calculator is provided to estimate the phase offset from the phase measurements.